Thermal tolerance windows were investigated in lugworms Arenicola marina (L.) from the North Sea, the English Channel and the Atlantic during winter and summer seasons. We adopted the concept of oxygen-and capacity-limited thermal tolerance and tested a variety of methods for their suitability to identify characteristics of the respective thermal window, such as critical, pejus (lat.: getting worse) and optimum temperatures (T c , T p and T opt ), as well as temperature-dependent performance. First, the effect of acute temperature changes on oxygen consumption and pumping activity was examined in artificial burrows ventilated by lugworms collected in winter and summer from North Sea mudflats (acclimation temperatures 5 and 10°C). Second, the window of acutely temperaturedependent growth was evaluated by following the integration of uniformly labelled 13 C-L-phenylalanine into proteins of the cuticulo-muscular tube, using nuclear magnetic resonance spectroscopy, possibly providing access to T p values. Third, seasonal changes in haemoglobin half-saturation oxygen tension (P 50 ) and haemoglobin oxygen status in vivo were studied in lugworms from the North Sea population. T c values were defined by use of oxygen consumption measurements, whereas pumping activity, growth performance and tissue oxygenation allowed determination of T opt . Our results show that the thermal window of actively ventilating lugworms is narrower in winter than in summer. The acute optimum temperature of growth performance was tilted towards the lower T c and likely shifts with acclimation. Methods were found suitable to identify the acute thermal window of oxygen-limited thermal tolerance in the lugworm. Consecutive comparison of populations during various seasons in a latitudinal cline should thus lead to a better understanding of the impacts of climate change on marine ecosystems.

Global warming trends in the marine environment currently lead to poleward shifts in the distribution of marine fauna along European coastlines indicating limited thermal tolerance of affected species and potential loss of their southernmost populations. The present study analyses the degree and limits of thermal specialisation in various populations of a key species of the intertidal zone, the lugworm Arenicola marina, which is exposed to highly fluctuating conditions in temperature, salinity, pH and oxygen levels during the seasonal as well as the tidal cycle. Thermal windows of tolerance and exercise performance were compared in three populations from the Russian White Sea, the German North Sea and the French Atlantic coast. Digging experiments in natural marine sediments quantified performance capacity and its optimum temperature. Temperature dependent ventilation and respiration were analysed in lugworms dwelling in artificial self-perfused burrows. Haemoglobin oxygen affinity was examined for an evaluation of population specific adaptations in the oxygen supply system. The results reflect a shift of both the thermal window and the thermal optimum towards higher temperatures with decreasing latitude. This shift was accompanied by a reduction of performance amplitude, i.e. the absolute number of digging periods, and a widening of the distance between critical temperatures, i.e. the total aerobic window of thermal tolerance.

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